A Dobsonian telescope is an altazimuth-mounted Newtonian telescope design popularised by John Dobson starting in the 1960s and credited with vastly increasing the size of telescopes available to amateur astronomers. The design is optimised for visually observing faint, deep-sky objects such as nebulae and galaxies.
Dobsonian telescopes are incredibly good light gatherers and are great for amateurs and professional astronomers alike. They are also very economical compared to other telescopes. The optical part of the telescope or OTA (Optical Tube Assembly) is the same as a Newtonian reflector telescope.
From its inception, telescope makers have been modifying the Dobsonian design to fit their needs. The original design fit the needs and available supplies of one person—John Dobson. Other people devised variants that fit their own needs, abilities, and access to parts. This has led to significant diversity in “Dobsonian” design.